Your CYA is 30 (there is no "28" on the tube, always round up) so that is now perfect to SLAM, and your Shock FC level is now 12. Be sure to stay at or above that (I would shoot for 15 or so to make up for loss)................... I have backwashed several times, trying to keep my media clean which provided a bit of a water change. Learning how to use my TFP test kit 8/12/15: CH 875, CL 2-4, PH 7.8, FC 20, CC .05, TA 70, CYA 28.
What is the "CL" reading? I thought that it was maybe from the OTO test, but your TC is 20.5ppm (if FC=20.0 & CC=0.5ppm) so the OTO should be off the scale (dark amber).
Also, pH is not reliable over a FC level of 10ppm so no need to check/adjust that during the SLAM.
Focus on FC & CC.
I can't help with the CH, but I know that there are others on the board that maintain a pool with high CH so don't worry about that right now either.
Focus on completing the SLAM.
Dom